Wireless communication devices such as terminals are also known as e.g. User Equipments (UE), mobile terminals, wireless terminals and/or mobile stations. These terms will be used interchangeably hereafter.
Wireless communication devices are enabled to communicate wirelessly in a wireless or cellular communications network or a wireless communication system, sometimes also referred to as a cellular radio system or a cellular network. The communication may be performed e.g. between two wireless communications devices, between a wireless communications device and a regular telephone and/or between a wireless communications device and a server via a Radio Access Network (RAN) and possibly one or more core networks, comprised within the cellular communications network.
Access nodes, such as base stations, communicate over the air interface operating on radio frequencies with the wireless communications devices within range of the base stations. In the context of this disclosure, the expression Downlink (DL) is used for the transmission path from the base station to the wireless communications devices. The expression Uplink (UL) is used for the transmission path in the opposite direction i.e. from the wireless communications devices to the base station.
Further, each base station may support one or several communication technologies. Examples of wireless communications technologies are New Radio (NR), Long Term Evolution (LTE), Universal Mobile Telecommunications System (UMTS) and Global System for Mobile communications (GSM).
In a forum known as the Third Generation Partnership Project (3GPP), telecommunications suppliers propose and agree upon standards for networks and investigate enhanced data rate and radio capacity.
Mobility
Mobility is a requirement in many wireless communications networks. A wireless communication network comprising base stations serving cells, may support mobility of a wireless communications device, i.e. service continuity of the wireless communications device, e.g. by transferring a connection between the wireless communications network and the wireless communications device from one cell to another cell or from one base station to another base station, commonly referred to as handover.
Handover
Handover is an important process of any wireless communications network where mobility is supported. With the handover the wireless communications network tries to assure service continuity of a wireless communications device by transferring a connection between the wireless communications network and the wireless communications device from one cell to another cell and/or from one access node to another access node, depending on the technology used. The handover should be executed without any loss of data and with as small interruption as possible.
FIG. 1a illustrates a schematic handover procedure in a prior art wireless communications network 101. The wireless communications network 101 comprises access nodes, including a first base station 111 and a second base station 112. In FIG. 1a a wireless communications device 140 moves away from the coverage of the first base station 111 and into the coverage of the second base station 112. In this scenario a handover of the wireless communications device 140 may be triggered if the wireless communications device 140 experiences a poor performance of a radio link to the first base station 111. For example, the wireless communications device 140 may trigger a handover event if it finds a new cell that is better than it's current cell. Thus, a comparison between the cells may be made. The network may then decide if handover shall be done or not.
When and to what cell and/or access node a handover occurs depends on several factors such as signal strength of reference signals, load conditions in the cells, service requirements of the wireless communications device 140, etc. The provision of efficient/effective handovers, e.g. described by minimum number of unnecessary handovers, minimum number of handover failures, minimum handover delay, etc., would affect not only the QoS of the end user but also the overall capacity and performance of the wireless communications network.
Thus, to enable a handover, it is necessary to find a suitable target cell and its base station, and to ensure that it is possible to sustain reliable communication with that target cell or base station. Candidates for suitable target cells are usually stored in so-called neighbor lists, which are stored in all base stations. To make sure that it is possible to sustain reliable communication with the target cell, the connection quality in the target cell needs to be estimated before the handover may be executed.
Handover in Existing Technology (Standardized in 4G/LTE)
In LTE, handover controlled by the wireless communications network and assisted by the wireless communications device 140 is utilized, for example as described by 3GPP TS 36.300 version 14.0.0. The wireless communications device 140 is moved, if required and if possible, to the most appropriate cell that assures service continuity and quality.
The quality in the target cell may be estimated by measurements related to the wireless communications device 140. Both downlink or uplink measurements may be considered when evaluating the target cell. In legacy wireless communication networks, such as GSM, WCDMA, LTE and WiFi, handover based on downlink measurements has been the most common solution. In those wireless communication networks handover based on downlink measurements is a natural solution as all base stations continuously transmit pilot signals that wireless communications devices in neighbor cells may use to estimate the target cell quality. This leads to that it is possible to estimate the quality of neighbor cells with relatively good accuracy.
With regards to prior art wireless communications networks, e.g. based on LTE, network energy consumption and network load may still be improved. For example, consistently broadcasted reference signals used for handover contributes significantly to the energy consumption and the load of the network.
Further, there is also room for improvements related to delays related to the handover.
5G, i.e. 5th generation mobile networks or 5th generation wireless communication networks, denotes the proposed next major phase of mobile telecommunications standards beyond the current 4G/International Mobile Telecommunications-Advanced standards.
One key design principle currently under consideration for 5G wireless communications networks is to avoid “always on signals”, i.e. consistently broadcasted, from the network as much as possible.
Beamforming
In order to overcome a coverage loss that occurs when a carrier frequency increases for 5G networks or systems, arrays of antenna elements may be employed to improve the coverage. This also gives a possibility to beamform a radio signal in certain spatial directions, or radio beams. That is, a radio beam is a radio signal transmitted in a certain direction and with a certain width. In the following, the expression beam will be used interchangeably with the expression radio beam.
DL Measurement Based Handover in Advanced Networks Using Beamforming
As mentioned above, modern wireless communication networks may use advanced antenna systems to a large extent. With such antennas, signals may be transmitted in narrow beams to increase signals strength in some directions, and/or to reduce interference in other directions.
Continuously transmitting pilot signals in all these beams is then less attractive, since it will generate much interference and also increase the base station energy consumption.
During a handover procedure in such a modern wireless communication network, maintenance of good Signal to Noise Ratio (SNR) and high bit rates may require that the wireless communications device 140 is handed over from one beam to another. In addition to a higher pathloss for the NR frequency bands, the higher frequencies also imply a more challenging propagation condition of radio signals in terms of lower diffraction and higher outdoor and/or indoor penetration losses. Thus the suitability of a certain beam may be quite sensitive to rather small movements and even rotations of the wireless communications device 140. Hence, which beam to hand over the wireless communications device 140 to may not be easily determined and to support handover between beams, the wireless communications device 140 has to perform a beam finding procedure. During such a beam finding procedure the radio access nodes that are potential target nodes for the handover, i.e. candidate access nodes, transmit DL beams identified by downlink signals, e.g. synchronization and/or reference signals, for the wireless communications device 140 to measure on. The beams are typically sequentially transmitted in a manner usually referred to as a beam sweep. The beam sweep may be continuously repeated or activated on demand. The wireless communications device 140 searches for the signals transmitted in the beams in the beam sweep and measures their respective quality. The beam with the best measured quality is typically selected as the target for the handover.
UL Measurement Based Handover
In a wireless communication network with advanced antennas, it becomes more attractive to rely on uplink measurements. Even wireless communication networks of today may rely on uplink measurements. For example, the wireless communications device 140 may transmit some uplink signal and several network nodes measure on that signal. The uplink signal may be a sounding signal, a reference signal or a combined synchronization and reference signal.
One reason that makes the UL measurement based handover more attractive in wireless communication networks with advanced antennas, capable of and, in high frequencies, relying heavily on advanced beamforming, is the difference in the UL and DL link budget. Since an access network node, such as the first base station 111 or the second base station 112, typically has more antennas and more advanced antenna configurations and a more advanced receiver than the wireless communications device 140, the receiver gain in the access network node is higher than in the wireless communications device 140. This makes the link budget more favorable in the uplink. For this reason, beamforming of the received uplink signal used for UL measurement based handover is not as crucial as for DL measurement based handover using DL beam sweeps as described above.
Typically, a single omnidirectional uplink signal transmission or possibly a beam sweep consisting of a few wide UL beams suffices to reach and provide a measurement opportunity for all the candidate access network nodes, since the beamforming gain is provided by the access network node, such as the second base station 112.
An UL measurement based handover may start by initiating uplink signal transmission from the wireless communications device 140, so that the candidate access nodes may measure on these uplink transmissions. The measurements of the quality of the received UL signal from the wireless communications device 140 from all the relevant cells and/or access nodes are collected and compared. Then the network decides on a suitable target cell and/or access node and the decision is communicated to the wireless communications device 140.
Timing Advance Acquisition
In many wireless communication networks it may be necessary for efficient operation that the transmissions from multiple wireless communications devices arrive at the access node, such as the first base station 111, in a synchronized manner. To enable such reception synchronicity, each wireless communications device has to take the propagation delay between the wireless communications device and the access node, or more precisely: the antenna(s) of the base station 111, into account when transmitting in the uplink.
The reference that the wireless communications device 140 uses when determining the timing of its uplink transmission is the timing, i.e. synchronization, of receptions of downlink signals. The UL transmission timing may be calculated by applying a so called Timing Advance (TA) to the downlink reception timing, such that uplink transmissions are initiated a time TA before the reference downlink synchronization, thereby ensuring that the uplink transmissions arrive at the first base station 111, e.g at the antenna site, at the expected times, e.g. aligned with frame and/or subframe and/or timeslot borders of the base station 111. The TA may be defined as the propagation delay between the base station and the wireless device and back to the base station again, i.e. TA=PDL+PUL where PDL is the downlink propagation delay and PUL is the uplink propagation delay.
To calculate the proper timing advance for the wireless communications device 140 which depends on its current position and distance to the base station, the base station 111 and the wireless communications device 140 need to cooperate, e.g. as follows. The typical way is to use a so-called random access (RA) procedure. Using UL transmission resources allocated for this purpose the wireless communications device 140 transmits an UL signal, often referred to as a random access preamble, that is easily detected, due to good correlation properties, by the receiving base station.
The wireless communications device 140 uses receptions of downlink transmissions from the base station 111 to determine the timing of the transmission of the UL signal. The first base station 111 measures the time of reception of the uplink signal in relation to the ideal timing. For example in relation to frame/subframe/timeslot borders. The ideal timing is e.g. the expected timing when the propagation delay is zero.
Based on this measuring, the first base station 111 calculates the appropriate TA for the wireless communications device 140 to use for subsequent uplink transmissions and communicates this TA to the wireless communications device 140, e.g. in a Random Access Response message.
The first base station 111 may subsequently measure the reception timing of further UL transmissions from the wireless communications device 140 and based on this continuously adapt the TA of the wireless communications device 140 using control signaling.
FIG. 1b illustrates how the wireless communications device 140 takes the propagation delay between the wireless communications device 140 and an access node, such as the first base station 111, into account when transmitting in the uplink.
First the first base station 111 sends 101 a synchronization signal. The wireless communications device 140 obtains time synchronisation with respect to the first base station 111 with delay T due to the propagation delay between the wireless communications device 140 and the first base station 111. Time synchronisation is also referred to as synchronisation and sometimes also referred to as DL synchronisation herein.
Obtaining time synchronisation with the first base station 111 means that the wireless communications device 140 obtains information that allows it to accurately know when a start of a symbol and a start of a subframe and/or a Transmission Time Instant (TTI) occurs in received DL transmissions. For example, in order to obtain synchronisation with a DL transmission, the communication device 240 may search for a known signal pattern, e.g. a symbol or sequence of symbols, in the received radio signal by correlating the known signal pattern with the received radio signal so that a location of a correlation peak in a time domain may be determined, e.g. with sufficient accuracy.
The wireless communications device 140 may then adjust its internal timing to match the timing of the first base station 111. This may further be referred to as the communication device 140 follows the timing of or a timing reference associated with the first base station 111.
Then the wireless communications device 140 sends 102 a random access preamble with a timing of the first base station 111 after a predefined time interval P1. For example, the wireless communications device 140 may send the signal such that a start of a symbol and a start of a subframe and/or a TTI is aligned with the start of the symbol and the start of the subframe and/or the TTI in received DL transmissions. This may also be described such that the communication device 140 follows the timing or a timing reference of the first base station 111.
P1 may be configured in the wireless communication device 140 by the first base station 111. But the wireless communication device 140 may also acquire this information from system information broadcast by the second base station 112.
The random access preamble arrives at the first base station 111 with an offset corresponding to 2*T from the reference timing at the first base station 111.
The first base station 111 estimates 103 a TA based on the offset. The first base station 111 signals the TA to wireless communications device 140.
The wireless communications device 140 applies 104 the received TA to its transmission timing.
The wireless communications device 140 sends 105 data after time interval P2-TA, and the data arrives at the first base station 111 with a correct reference timing, i.e. with the reference timing of the first base station 111.
The wireless communications device 140 typically has to perform a random access procedure in conjunction with initial network access, including transition from an energy saving state, e.g. idle mode, to a connected/active state, after a handover to a new cell and/or base station. The same applies when the wireless communications device 140 has gone long enough without transmitting in the uplink to risk having lost its uplink synchronization, i.e. the TA cannot be trusted as valid anymore, which may occur due to movements of the wireless communications device 140.